Implement Cast Streaming mirroring, DLNA casting, daemon+GUI, and breadd integration

Builds out the full v1 scope: a vendored+patched openscreen subset for
low-latency Cast Streaming (Mirroring receiver 0F5096E8) alongside the
existing Cast V2/HLS and new DLNA/AVTransport casting paths, breadcastd's
Idle/Casting state machine with a private IPC socket, the breadcast GTK4
popup as a thin IPC client, and bread.cast.*/bread.command.cast.* breadd
integration (device discovery, start/stop, mirroring lifecycle events).
Also adds bakery/systemd/Forgejo CI packaging.

Validated end-to-end against a real Chromecast/Google TV: negotiated
Cast Streaming session, live pipeline playback, and daemon+GUI click-to-cast/
stop through the actual popup.

breadcast-caststream-sys/vendor/openscreen/PATCHES.md

@@ -0,0 +1,64 @@
+# Vendoring notes
+
+This directory is a **pruned subset** of [chromium/openscreen](https://chromium.googlesource.com/openscreen),
+pinned at the commit in `PINNED_COMMIT`, licensed BSD-3-Clause (see `LICENSE`).
+It contains only the files needed for the Cast Streaming *sender* data path
+(RTP/RTCP send, OFFER/ANSWER message types, AES frame encryption) plus the
+POSIX platform/util plumbing that path needs — not the receiver, not OSP
+discovery, not remoting/RPC, not TLS (breadcast's TLS CASTV2 control channel
+is handled by the existing `rust_cast`-based Rust code; this vendored code
+only drives the UDP RTP/RTCP data path once `rust_cast` has already
+negotiated OFFER/ANSWER and we know where to send packets).
+
+It is compiled directly via the `cc` crate in `../build.rs`, not GN/Ninja —
+there is no build system here to regenerate anything from.
+
+## Rolling the pin
+
+There are no release/API-stability guarantees upstream. To update:
+1. Re-run the file selection against the new commit's `cast/streaming/BUILD.gn`
+   (`:common` + `:sender` targets, minus `public/sender_session.*` and
+   `public/rpc_messenger.*` — see "What's excluded" below) plus
+   `platform/BUILD.gn`'s `:base`/`:api`/`:standalone_impl` targets.
+2. Re-apply the patches below (they're small; check if upstream has since
+   fixed the same problem and the patch can be dropped).
+3. Update `PINNED_COMMIT` and rebuild.
+
+## What's excluded and why
+
+- **`public/sender_session.{h,cc}`, `public/rpc_messenger.{h,cc}`,
+  `public/receiver_session.*`, receiver-side files, `remoting.proto`/
+  `input.proto`** — `SenderSession` bundles mirroring negotiation together
+  with RPC/remoting/input support, which pulls in protobuf
+  (`input.pb.h`/`remoting.pb.h`) for no benefit here (breadcast only ever
+  does one-way video mirroring). Instead, `../src/session.cc` drives
+  `Offer`/`Answer`/`SessionConfig` directly — logic adapted from
+  `sender_session.cc`'s `CreateMirroringOffer`/`StartNegotiation`/
+  `SelectSenders`, minus everything RPC/remoting/audio/input-related.
+- **All TLS support** (`platform/impl/tls_*`, `platform/impl/stream_socket*`) —
+  unused; see above.
+- **`util/crypto/{certificate_utils,digest_sign,pem_helpers,rsa_private_key,
+  secure_hash,sha2}.*`** — only needed for OSP discovery / X.509 certificate
+  handling, not the RTP data path.
+- **`util/scoped_wake_lock_mac.cc`** — macOS-only.
+
+## Local patches (not upstream)
+
+1. **`build/build_config.h`, `build/buildflag.h`** — stand-ins for Chromium's
+   GN-generated versions of these headers. Only define what the 3 callers
+   here actually check (`IS_POSIX`, `IS_LINUX`, `IS_APPLE`, `IS_ANDROID`),
+   hardcoded for Linux.
+2. **`patches/aes_ctr128_compat.cc`** — `frame_crypto.cc` calls
+   `AES_ctr128_encrypt()`, a BoringSSL convenience wrapper not in system
+   OpenSSL's public headers. Reimplemented from scratch (standard CTR mode
+   over `AES_encrypt()`, which OpenSSL does still expose).
+3. **`platform/impl/platform_client_posix.{h,cc}`** — stripped the
+   `TlsDataRouterPosix` member/accessor (see "All TLS support" above).
+4. **`util/crypto/openssl_util.{h,cc}`** — dropped `SSLErrorCodeToError()`/
+   `GetSSLError()`, which reference BoringSSL's `SSL_error_description()`
+   (not in system OpenSSL). Unused for the same reason as (3).
+5. **`util/base64.cc`** — upstream implements this on
+   `third_party/modp_b64`, which isn't fetched by a plain shallow clone
+   (pulled in separately via gclient/DEPS in a full Chromium checkout).
+   Reimplemented on `EVP_EncodeBlock`/`EVP_DecodeBlock` from system OpenSSL
+   instead, same public interface.